Nanoparticles are small objects having diameters less than 1 micron that behave as a whole unit with respect to transport and properties. Nanoclusters are nanoparticles having at least one dimension between 1 and 10 nanometers and a narrow size distribution. Metal nanocatalysts are high surface-to-volume metal nanoparticles, particularly metal nanoclusters, useful for catalyzing many different reactions. Metal nanocatalysts have higher catalytic activity than bulk metal structures comprising the same metal, and may be synthesized in a variety of shapes that can have an effect on their relative activities. The preparation of metal nanocatalysts in the form of nanoclusters where the size, morphology and properties of the nanoclusters may be controlled by designed synthesis may have applications in a variety of high technology fields such as sensing, bioanalysis, biological labeling and semi-conductors. However, control of the size and morphology of particles and the nanoscale level is challenging.
Several methods have been developed recently to control the synthesis of metal nanoclusters including incipient wetness impregnation, electron beam lithography, sol-gel, evaporation methods, coprecipitation and colloidal synthesis with block copolymers or dendrimers. The methods leading to the most reliable size distribution (near-monodisperse) are electron beam lithography, evaporation methods onto oxide support and colloidal synthesis. However, electron beam lithography must be performed in a very controlled environment leading to a very expensive material. This is also the case with evaporation methods in which the very controlled environment involves ultra-high-vacuum. The colloidal approach may be performed with a polymer matrix allowing production of nanoparticles with a monodisperse size distribution, but the metal nanoclusters so produced are synthesized within a polymer shell which limits catalytic activity as 100% of the surface of the nanocluster is in contact with the polymer. The colloidal method also generally requires the use of a reducing agent to obtain a metal nanocluster from metal salt precursors incorporated inside the matrix.
In colloidal synthesis, the most common method of reducing metal salt precursors is by chemical reduction including alcohol reduction, hydrogen reduction and sodium borohydride reduction. Other reduction techniques include electrochemical, photochemical and sonochemical methods. These methods are often expensive and not environmentally friendly. The use of reducing agents may also reduce the efficiency of the catalysts.
In addition, macromolecular crowding and spatial confinement has been shown to enhance reactions depending on the relative sizes and shapes of the concentrated crowding species and on the diluted reactants and products. In general macromolecular crowding is expected to increase reaction rates that are slow, transition state-limited association reactions, and decrease the reaction rate of fast, diffusion-limited association reactions. For example, spatial confinement physically restricts the available conformations that a protein can form, which can make the folded state more favorable. Macromolecular crowding can also yield the same outcome but since the boundary is not rigid, more conformations are available. Another consideration is the electrical properties of the confining media. The size and shape of the confining body can lead to a reciprocal optimization of the van der Waals interaction between the molecules and the structure. This non-covalent interaction can lead to conformational changes that increase catalytic activity in zeolites and the reduction of iron oxide in carbon nanotubes, for example.
There remains a need for environmentally friendly methods of synthesizing monodisperse metal nanoclusters that are catalytically active and available for use as a catalyst.